Many types of electronic equipment such as portable telephone devices, PDAs (Personal Digital Assistant) terminals, and notebook personal computer terminals incorporate printed wiring boards that package many electronic components within a limited space. However, advances in achieving smaller size and lighter weight in such electronic equipment have improved the portability, and the potential for being dropped during transport has therefore become extremely high. When the impact force (impact force of a fall) in such falls is applied to the electronic apparatus, the protection of electronic components on the printed wiring board from the impact force of the fall is of crucial importance from the standpoint of the reliability of the electronic apparatus.
At present, various solutions have been proposed as constructions for protecting electronic components on a single printed wiring board incorporated in an electronic apparatus from the impact force of a fall, such as increasing the rigidity of the printed wiring board itself, or reinforcing the securing points between the printed wiring board and the case of the electronic apparatus terminal and increasing the number of these securing points. Additional solutions that have been proposed include the addition of a shock-absorbing material to the entire surface of the printed wiring board or to the securing points between the printed wiring board and electronic apparatus terminal case, and the addition of shock-absorbing material to points of contact between electronic components on the printed wiring board and the electronic apparatus case.
FIG. 1 shows the mounting construction disclosed in Patent Document 1 (JP-A-2003-304081) that has been conventionally adopted for installing a printed wiring board.
This is a construction in which a board-installing tool equipped with housing 1904 and two board support members 1901 secured to housing 1904 is prepared to install printed wiring board 1905 on which LSI package 1908 is mounted. Printed wiring board 1905 is inserted and held in an arched form between board support members 1901 and thus installed in housing 1904.
FIG. 2 shows the mounting construction of a printed wiring board that has been conventionally adopted and disclosed in Patent Document 2 (JP-A-2002-151866).
In this construction, through slot-holes 2004 are provided at positions of printed wiring board 2003 that correspond to each boss 2006 having lower tapped hole 2002 in each corner of case 2001. Printed wiring board 2003 is then mounted on case 2001 by fastening screws 2005 by way of each through slot-hole 2004 to lower tapped hole 2002 of each boss 2006. As a result, although the application of outside mechanical stress case 2001 may cause deformation of case 2001, screws 2005 and bosses 2006 can slide along the slots in the direction in which the slots of through slot-holes 2004 were cut, whereby a configuration is realized for either eliminating the amount of deformation of printed wiring board 2003 or making the amount of deformation of printed wiring board 2003 less than that of case 2001.
FIG. 3 shows the mounting construction disclosed in Patent Document 3 (Japanese Patent No. 3698091) that has been conventionally adopted for mounting a printed wiring board.
In this construction, when circuit board 2101 is held inside apparatus case 2102, the left side of circuit board 2101 is secured to apparatus case 2102 with circuit board 2101 suspended inside apparatus case 2102. A pair of shock-absorbing members 2105 and 2106 is then disposed between the upper and lower surfaces of this circuit board 2101 and the corresponding upper and lower surfaces inside apparatus case 2102, whereby circuit board 2101 is interposed and held between the pair of shock-absorbing members 2105 and 2106. Thus, even if an impact force applied to apparatus case 2102 is conveyed to circuit board 2101, the pair of shock-absorbing members 2105 and 2106 disposed on both the upper and lower surfaces of circuit board 2101 can limit any warp deformation of circuit board 2101. In addition, the shock conveyed to circuit board 2101 can be mitigated by the pair of shock-absorbing members 2105 and 2106. As a result, a construction is realized in which an impact force applied to apparatus case 2102 is not conveyed, as is, to circuit board 2101 and this construction can thereby prevent damage to circuit board 2101.
Problems inherent to the above-described background art are next considered.
The electronic components on a printed wiring board are normally electrically and mechanically connected to the electrical wiring that is formed on the printed wiring board by using an electrical connection material such as solder. As a result, when the impact force of a fall is applied to the printed wiring board, the electrical connections such as solder are subjected to excessive stress, whereby defects such as the breakage or peeling of the electrical connections may occur, and these defects are a primary factor in greatly reducing electrical and mechanical reliability. Regarding the behavior of the printed wiring board at such times, the impulsive external force resulting from the impact force of a fall is applied to the printed wiring board, and due to this impulsive external force, the printed wiring board bends greatly in the same direction as the direction of application of the impact force of the fall (initial amplitude). The printed wiring board then undergoes damped vibration (residual vibration) at the natural frequency of the printed wiring board, and then comes to rest. When a defect occurs in a solder connection due to a single fall, it is thought that this defect is generated as follows. The large deformation resulting from the initial amplitude of the printed wiring board causes a large distortion in the interface of the electrical wiring and solder bonding layer (alloy layer) formed on the printed wiring board that exceeds the elastic stress limit, leading to brittle fracture of the solder connection. On the other hand, when a problem does not occur due to a single fall but a defect occurs in solder connections due to a succession of falls, the defect is believed to be generated as follows. The repeated vibrations resulting from the residual vibration of the printed wiring board cause the repeated generation of distortion that causes at least a particular fixed stress within the elastic stress limit in the solder material or in the interface of the electrical connections and the solder bonding layer (alloy layer) formed on the printed wiring board, leading to the fatigue fracture of the solder connections.
Essentially, protecting the electronic components on a printed wiring board from the impact force of a fall requires both the reduction of the initial amplitude as well as the decrease of the frequency of residual vibration and realization of early attenuation.
As previously stated, when the impact force of a fall is applied to an electronic apparatus, the protection of the electronic components on the printed wiring board from the impact force of the fall is of crucial importance from the standpoint of the reliability of the electronic apparatus. For this purpose, increasing the rigidity of the printed wiring board itself, reinforcing the securing points between the printed wiring board and the electronic apparatus terminal case, and increasing the number of securing points have all been proposed as constructions for protecting the electronic components on one printed wiring board that is mounted in an electronic apparatus from the impact force of falls. In addition, adding shock-absorbing material to the entire printed wiring board or to the securing points between the printed wiring board and the electronic apparatus terminal case and adding shock-absorbing material to the points of contact between the electronic components on the printed wiring board and the electronic apparatus terminal case have also been proposed.
When measures are adopted such as increasing the rigidity of the printed wiring board itself, reinforcing the securing points between the printed wiring board and the electronic apparatus terminal case, and increasing the number of securing points as previously described, the initial amplitude can be reduced by increasing the spring constant of the vibration system. However, as an unintended consequence, this approach results in an increase in the natural frequency of the printed wiring board and no change in the attenuation factor of the vibration system, whereby the wave number of the residual vibration actually increases.
In addition, adopting measures such as adding shock-absorbing material to the entire surface of the printed wiring board or to the securing points between the printed wiring board and the electronic apparatus terminal case results in a reduction of the natural frequency of the vibration system due to the shock-absorbing material, and further, an increase in the attenuation factor, thereby achieving a decrease of the wave number of residual vibration and early attenuation. In this case, however, the spring constant remains unchanged and the initial amplitude therefore cannot be sufficiently reduced.
In sum, none of these measures is capable of achieving both a reduction of initial amplitude as well as a reduction of the wave number of residual vibration and early attenuation.
In the construction disclosed in Patent Document 1 (JP-A-2003-304081), a printed wiring board is installed in a housing that is in the shape of an arch. This construction is equivalent to increasing the rigidity of the printed wiring board itself. As a result, the initial amplitude can be decreased by increasing the spring constant of the vibration system as previously described, but the natural frequency of the printed wiring board increases, and the attenuation factor of the vibration system remains unchanged, whereby the wave number of the residual vibration, instead of decreasing, becomes greater.
In the construction disclosed in Patent Document 2 (JP-A-2002-151866), if deformation of a case is accompanied by deformation of a printed wiring board that is incorporated in the case that slowly becomes deformed and this deformation approaches a substantially dead weight, the exhibited effect is the maintenance of electrical and mechanical reliability. However, when impulsive outer force such as the impact force of a fall is applied to the case, the printed wiring board is not able to slide along the slot in the direction in which the slots of the through slot-holes have been cut. The resulting situation is therefore no different than if the printed wiring board were fastened by screws to the case.
The construction disclosed in Patent Document 3 (Japanese Patent No. 3698091) is a construction in which both the upper and lower surfaces of a circuit board are held by a pair of shock-absorbing members disposed between the upper and lower surfaces inside an apparatus case, whereby the shock-absorbing material lowers the natural frequency of the vibration system and the raises the attenuation factor. Accordingly, a reduction of the wave number of residual vibration and early attenuation are possible. However, to obtain a sufficient attenuation factor, a material having a low modulus of longitudinal elasticity (Young's modulus) must be selected as the shock-absorbing material. Because the spring constant of the vibration system does not change in this case, a sufficient reduction of the initial amplitude is not possible. In addition, because this is a construction in which both the upper and lower surfaces of the circuit board are held interposed between the pair of shock-absorbing members disposed between the upper and lower surfaces inside the apparatus case, the space between the circuit board and the case is occupied by the shock-absorbing members. As a result, this construction is unable to cope with increases in circuit scale that accompany higher functionality and greater multifunctionality, and further, does not allow smaller and lighter constructions.
As previously stated, none of the constructions for mounting a printed wiring board that have been adopted conventionally was capable of achieving both a reduction of the initial amplitude, and early attenuation of residual vibration as well as a reduction of the wave number of residual vibration. Consequently, these constructions were incapable of protecting electronic components on a printed wiring board from the impact force of a fall and were incapable of guaranteeing reliability of the electronic apparatus, and moreover, were incapable of achieving smaller size and lighter weight.